In such systems, which are to enable a driver approaching a vehicle to be recognized remotely, an on-board module integrated in the car sends out an interrogation signal to an "electronic label" which is generally implemented in the form of a badge or a flat card worn or carried by the driver, e.g. in a pocket. The card receives the interrogation signal, analyzes it by means of a microprocessor which is incorporated in the card, and generates an encoded identification (ID) signal specific to the card, which signal is transmitted back from the card to the on-board module which then performs various actions, e.g. unlocking the doors, and automatically adjusting the positions of the rearward mirrors, the steering wheel, etc.
One of the main difficulties encountered with this type of remote control lies in the fact that the card carried by the driver must be small in size (typically credit card sized) while nevertheless remaining operational for a long period of time (about one year), and as a result the energy consumption of the circuits it contains is a major constraint governing its design.
In addition, since identification must be capable of taking place "on the fly", i.e. without contact between the on-board module and the card and without even requiring the driver to take the card out of a pocket and press a button on the card, it is necessary for the card to be ready at an instant to receive and analyze an incident interrogation signal without any prior external action being taken by the driver (e.g. pressing a button on the card).
Further, given that the microprocessor consumes energy at a relatively high rate, it cannot be left switched on permanently in its normal active state. Use is thus made of the possibility of switching the microprocessor back and forth between an "awake" state in which it is active and a "sleep" state in which its consumption is tiny or zero. The microprocessor is switched from the sleep state to the awake state by applying a wake-up signal to an input of the microprocessor specially designed for this purpose, or else merely by applying the signal to the power supply terminal of the microprocessor (in which case the wake-up signal is constituted by the power supply direct voltage).
It can thus be seen that only a receiver circuit needs to be powered permanently (which receiver circuit transforms the received radiofrequency signal into a demodulated and amplified low frequency signal), together with an interface circuit disposed between the receiver and the microprocessor and capable of verifying that the demodulated signal satisfies a certain number of criteria, and if it does, then applying the wake-up signal to the microprocessor in order to switch it to its active state.
The present invention relates specifically to such a circuit (referred to below as a "power supply wake-up circuit") providing the interface between the circuit for receiving remote controlled signals transmitted by the on-board vehicle and the microprocessor in the ID card.
It should nevertheless be observed that the application to remote control of cards is not limiting and that other comparable applications could be envisaged without going beyond the scope of the present invention.
Similarly, the power supply wake-up circuit of the invention is perfectly capable of processing signals other than those produced by a radio signal receiver, and the present invention is thus equally applicable to waking up the power supply of a microprocessor from input signals that satisfy comparable conditions but that are produced by circuits other than a radio receiver.
Also, the term "microprocessor" should not be understood in its narrow technological meaning, but in a wide sense covering any circuit capable of performing digital processing, and thus extending equally well to circuits having a high degree of integration (such as microcontrollers) and to circuits having a lower degree of integration (circuits that need to be associated with discrete components). In any event, the function performed by the circuit of the invention is completely independent of the technology and the structure of the circuits to which it is connected, both upstream and downstream.
The present invention thus provides a power supply wake-up circuit capable of discriminating an effective interrogation signal produced by the on-board module of a vehicle from other signals that may be picked up by the radio receiver, and most particularly mains frequency interference (e.g. 100 Hz) that could, under certain reception conditions and unless special precautions are taken, be misinterpreted as a control pulse train of an interrogation signal.
More precisely, in order to perform such discrimination, provision is made for the interrogation signal message (i.e. a digital message encoded in the form of pulses) to begin with a header constituted by a succession of similar pulses (referred to below as "control pulses"), with there being a predetermined number of these pulses and with these pulses having a predetermined recurrence frequency.
After discrimination, this message header is intended to wake up the microprocessor so that once it has switched to its active state it can process the following pulses of the coded message per se contained in the interrogation signal.
Firstly, in order to discriminate sporadic interference, a first condition is provided requiring the control pulse train to be constituted by a minimum number of successive pulses, e.g. not less than sixteen control pulses.
Secondly, in particular to avoid the microprocessor being woken up uselessly by mains frequency interference, a second condition is provided requiring the recurrence frequency of the control pulses to be not less than a predetermined limit frequency that is considerably higher than mains interference frequency (e.g. 100 Hz); it would thus be possible to provide a typical recurrence frequency of 1 kHz, corresponding to control pulses having a recurrence period T=1 ms.
In contrast, it is not desirable to lay down conditions on the duration of each pulse (i.e. on the duty ratio of the control pulse train) since this parameter may suffer from relatively large dispersion from one on-board module to another and drift may also occur as a function of temperature variations and of component aging.
That is why only the above-mentioned two conditions are taken into account simultaneously, i.e. a minimum number of control pulses and a minimum frequency for the control pulses.
Further, given the above-mentioned energy consumption constraints and the fact that the wake-up circuit must be capable of being operational at any moment, the energy consumption of the wake-up circuit itself must be as low as possible so as to have no harmful effect on the lifetime of the battery powering the ID card.
EP-A-0 343 619 describes a microprocessor wake-up circuit situated on-board a vehicle for responding to signals coming from a bade or the like that is suitable only for transmission. This wake-up circuit is subjected to much less severe constraints on its electrical power supply because of the presence of the vehicle's own battery, and it is also complex and unsuitable for integration in a small-sized portable badge.
U.S. Pat. No. 3,150,271 describes a transistor "pumping" circuit, however this document neither mentions nor suggests in any way that such a circuit should be used in co-operation with threshold means and with a radio receiver circuit, nor does it make any mention of an application to waking up a microprocessor in a portable badge.